[Support] Get process statistics in ExecuteAndWait and Wait

The functions sys::ExcecuteAndWait and sys::Wait now have additional
argument of type pointer to structure, which is filled with process
execution statistics upon process termination. These are total and user
execution times and peak memory consumption. By default this argument is
nullptr so existing users of these function must not change behavior.

Differential Revision: https://reviews.llvm.org/D78901